This invention relates to a web dryer with a variable ventilation rate, and more particularly to web dryers which are used in the application of solvent based coatings such as printing ink to webs of paper and the like, such as in the graphic arts industry.
A web dryer of the general type under consideration here is illustrated in the above-identified Whipple U.S. Pat. No. 4,414,757 which includes a dryer housing through which a web travels and which contains a plurality of web dryer nozzles, an air inlet, an air exhaust and a source of heat. The nozzles are often generally of the Coanda type, described more fully in said Whipple patent.
Of major importance in using such dryers in the graphic arts industry is the requirement of evaporating the solvents from the coatings disposed on the paper web. However, such solvents are often flammable and it is essential that a certain ventilation rate be maintained in the dryer so that the solvent concentration does not increase to the explosive or flammable point. This point is the leanest mixture of solvent and air which would cause an explosion, and is referred to in the industry as the "Lower Flammable Limit" or LFL.
Under the guidelines of the National Fire Protection Association (NFPA), and with only several minor exceptions, the vapors in a working dryer must be no higher than 25% of the said Lower Flammable Limit, and the industry, during the normal course of operation most often finds themselves operating at less than 10% of the LFL.
A number of problems have arisen in attempting to maintain the vapor levels at or near 25% of the Lower Flammable Limit. The usual procedure has been to provide an air ventilation rate which is based on the ink application machine operating at maximum design conditions. However, this is an inefficient procedure when operating at less than the maximum conditions, i.e. 25% of the LFL, in that fuel is used to heat unrequired quantities of ventilation air. Previous attempts to reduce or vary the ventilation rate have not been fully successful for a variety of reasons. It must be borne in mind that when operating at or near 25% of the LFL, undesirable solvent condensation may occur more easily on the surfaces within the dryer and may interfere with the web drying function.
Some prior systems have attempted to measure the "solvent humidity" in the dryer air at the exhaust duct and then vary the ventilation rate accordingly. See U.S. Pat. No. 2,743,529. These systems are believed to be unstable and the equipment has often required recalibration. Other prior systems have attempted to measure the oxygen content within the dryer atmosphere and then vary the ventilation rate, but a failure of such devices could lead to a catastrophic explosion. See also U.S. Pat. No. 3,909,953 wherein a damper is used to control the air supply or exhaust for the purpose of keeping the oxygen level low.
It is a task of the present invention to provide a unique system for varying the ventilation rate of a web dryer which will not be subject to the various disadvantages of the known dryers and which will substantially reduce the energy consumption thereof under many operating conditions, while also reducing the problem of solvent condensation within the dryer. It is a further task of the invention to provide a low degree of operator attention needed, consistent with the important requirements of safety and reliability. It is yet another task to provide a system which, while relatively sophisticated, can be retrofit to prior driers in the field.
In considering the invention it should be kept in mind that the dryer "box" or housing is normally maintained at a generally constant slightly negative pressure. The mass of air being exhausted is equal to the mass of air being supplied. The ventilation rate is, in effect, equal to the exhaust rate, which provides a measure thereof, so it is important to measure the latter for purposes of controlling the system. Air normally enters a dryer in a number of ways, the most obvious being: (a) through the narrow slots in the dryer wall through which the web passes, where air infiltrates, and (b) the heating (combustion) air inlet for drying. However, the mass of air infiltrating through the said web slots is normally not controllable, whereas the mass of said combustion air is. Furthermore, most air dryers of the type under consideration here are provided with a recirculating air system for purposes of maintaining the air temperature as constant as possible within the dryer itself.
In accordance with the various aspects of the invention, a special computation is made as to what the ventilation rate through the dryer should be, depending on certain conditions, and the ventilation rate automatically varied in accordance with the said conditions. This desired ventilation rate is equivalent to the rate of exhaust discharge.
In order to properly control the ventilation rate, it is necessary to know the mass of ink or other solvent based coating entering the dryer per unit of time, such as lbs./hr. This is dependent on the rate of ink application to the web at the printing press or other application device, either measured or calculated.
The measured ink application rate may be determined by sensing the total amount of ink flowing from one or more ink sources to the ink application device such as a printing press.
The calculated ink application rate may be determined by sensing the number of flowing ink supplies feeding onto the web at the printing press and calculating the amount of flowing ink supplied as if the ink was applied at a maximum amount (consistent with industry standards), such as in lb/sq. in. However, such sensing and calculating doesn't directly indicate the area (mass) of ink put down on the traveling web without taking additional conditions into account. In order to obtain the area measurement, it is therefore necessary to also measure the width of the web and the speed of web movement through the dryer.
The various aspects of the invention contemplate determining the ink application rate as per one of the above methods, feeding information regarding the actual exhaust flow (ventilation rate) and these determinations to a ventilation rate control, and then varying the exhaust flow in response thereto to maintain the desired ventilation rate through the dryer, as by controlling a variable speed exhaust blower.
In actuality, the ventilation rate includes the incoming combustion air and the infiltration of air through the web slots or other small openings. The difference between the ventilation rate and the other items mentioned must be provided by make-up air. Therefore, at the same time, make-up air is provided to the dryer interior, preferably at a point remote from the exhaust discharge duct so that it mixes with the recirculating air to keep the solvent vapor concentration inside the dryer uniform and to reduce potential solvent condensation problems.
The make-up air can be supplied from a number of sources. In embodiments where the amount of air infiltration at the web slots is reduced, an open make-up air opening may be disposed in the dryer housing wall. Alternately, a control loop responsive to internal dryer box pressure may control a valve or a variable speed blower associated with an open or blower fed make-up opening. Another alternative control loop arrangement feeds the make-up air in through a valved input to the combustion burner which heats the dryer air. Either control loop alternate performs the same function. They both maintain box pressure and thereby maintain reduced infiltration at the web slots, despite variations in exhaust flow or burner air flow. In a further embodiment, where control of the amount of air infiltration at the web slots is not contemplated, air entering the web slots is preheated and performs the make-up function.